Serotonergic synaptic input to facial motoneurons: localization by electronmicroscopic autoradiography

Serotonergic nerve terminals in the facial motor nucleus were labelled with [³H]5-hydroxy-tryptamine. When serotonergic nerve terminals were destroyed (by the selective neurotoxin 5,7-dihydroxytryptamine) the labelling was lost. By electron-microscopic autoradiography, labelled serotonergic terminals were found to make axo-dendritic or axo-somatic junctions with facial motor neurons. No axo-axonic junctions were observed.These morphological findings are consistent with physiological studies which indicate that 5-hydroxy-tryptamine facilitates the excitation of facial motoneurons through a direct postsynaptic action.